1. Field of the Invention
The present invention relates generally to the field of commercial blasting and more particularly to a new and improved package for blasting agents.
2. Description of the Prior Art
In modern commercial blasting operations such as strip mining or quarrying a series of blast holes are drilled in the material to be blasted and a blasting agent such as prilled ammonium nitrate and fuel oil (ANFO) is introduced into the hole. Among the factors which bear upon the efficiency with which such explosives may be used are the percentage volume of the blast hole actually loaded with blasting agent and the presence of water in the blast hole.
The volume energy of the ANFO blasting agent is maximized when the blast hole is substantially completely filled such that the blasting agent is in direct contact with the material to be blasted. Under ideal circumstances the ANFO is preferred to be loaded in bulk directly into the blast hole. If, however, the blast holes contain even small amounts of water it will be readily absorbed by the ANFO. The result is that the water replaces the air sites in the porous prill and reduces both the presence and the uniform distribution of fuel oil in the mix. The end result is to eliminate parially; substanially or totally the potential hot spot locations for adiabatic compression which are essential for the propogation of detonation in modern chemical explosives. To overcome the wet blast hole problem it therefore becomes necessary to load the blasting agent in waterproof packages which in the first instance must be of a small enough diameter to drop easily down the blast hole and in the second instance expandable on reaching the bottom such as to completely fill the blast hole cross-section while maintaining their water tight integrity.
Prior to the present invention various attempts have been made to provide a package for blasting agents which is useful in the wet blast hole environment. U.S. Pat. Nos. 1,543,851 dated June 30, 1925 to Holderer and No. 2,966,855 dated Jan. 3, 1961 to Barco are representative of the prior art. Holderer describes an explosive cartridge formed of an outer paper wrapper enclosing a larger diameter inner woven textile wrapper which may be waterproofed. The ends of the cartridge are tied closed. The outer wrapper of the Holderer cartridge cannot be relied upon to rupture upon impact with the bottom of the blast and therefore must be tamped in order to ensure that the blasting agent expands to fill the full cross-section. A second problem arises due to the high water pressures which may be encountered (e.g. approximately 20 psi at 40 feet). The tied closure of Holderer cannot reliably provide water tight integrity under such water pressure for a sufficient time period. Barco addresses the problem of package expansion upon impact but requires a positive retaining arrangement which must be released prior to dropping the package into the bast hole. Again, the tied closure has been found to be less than fully effective as a water-tight seal.